This job error suggests that the actual size of the Veeam Agent machine’s disk differs slightly from what is recorded in the metadata file, causing the backup to fail.
Rename or delete the metadata file, or delete and recreate the backup job. All of these actions will trigger a new metadata file (*.vbm) to be created.
Example Problem (from Veeam Case 04228176):
We have a new tenant Windows workstation that does not yet have a good backup. They frequently shut down the PC or allowed it to go to sleep, disrupting the initial backup over the course of several days. For an undetermined reason, Veeam is unable to keep the PC awake during the backups. Despite modifying the PC’s sleep settings to never sleep, restarting the Veeam service, and even rebooting the machine there appears to be a Backup Cache Sync operation that frequently kicks off (every ~10 minutes), but always fails to complete and transfer any data. When reviewing the logs, we see this job error.
[12.06.2020 12:53:26] <01> Job Error Disk [b90935eb-1d8b-4fb9-ab46-fc91b75114bf] processed offset  is bigger than disk capacity  at Veeam.EndPoint.CBackupCacheSyncSource.ProcessDiskDigests(IBackupTaskTarget target, CCachedDigestsInfo diskDigests, CEndPointSyncVbDiskBackupObject diskBackupObject, CEndPointDiskInfo diskInfo)
Veeam Support Response
This error means that the size of the actual Agent machine’s disk differs slightly from what is recorded in the metadata file which causes an issue when using the metadata to create a new incremental.
To resolve, you can recreate the job or alternatively, rename the metadata file, as this will trigger a new metadata file to be created.